LP3942 Datasheet, PDF (18/20 Pages) National Semiconductor (TI) – The Dual RGB LED Controller with 1.5x/2x Charge Pump and SPI Interface

Charge Pump Operational
Description (Continued)
regulation LP3942 output voltage can be approximated by:
V_OUT= 1.5 x V_IN – I_OUT x R_OUT. Again, this equation only
applies at low input voltage and high output current where
the LP3942 is not regulating. See Output Current vs. Output
Voltage curves in the Typical Performance Characteristics
section for more details.
On 2x mode the functionality is similar, only the output
voltage is set to 5.0V and out-of-regulation output voltage
can be estimated by: V_OUT = 2.0 x V_IN – I_OUT x R_OUT. Output
resistance is approximately same as in 1.5x mode.
THERMAL SHUTDOWN
The LP3942 implements a thermal shutdown mechanism to
protect the device from damage due to overheating. When
the junction temperature rises to 160˚C (typ), the part
switches into Startup mode. The LP3942 releases thermal
shutdown when the junction temperature of the part is re-
duced to 140˚C (typ). Thermal shutdown is most-often trig-
gered by self-heating, which occurs when there is excessive
power dissipation in the device and/or insufficient thermal
dissipation. LP3942 power dissipation increases with in-
creased output current and input voltage (see Power Effi-
ciency and Power Dissipation section). Because of auto-
matic recovery from thermal shutdown function, thermal
cycling is the typical result. Thermal cycling is the repeating
process where the part self-heats, enters thermal shutdown,
cools, turns-on, and then heats up again to the thermal
shutdown threshold. Thermal cycling is recognized by a
pulsing output voltage and can be stopped be reducing the
internal power dissipation (reduce input voltage and/or out-
put current) or the ambient temperature.
If thermal cycling occurs under desired operating conditions,
thermal dissipation performance must be improved to ac-
commodate the power dissipation of the LP3942. Fortu-
nately, the LLP package has excellent thermal properties
that, when soldered to a PCB designed to aid thermal dissi-
pation, allows the LP3942 to operate under very demanding
power dissipation conditions.
OUTPUT CURRENT LIMITING
The LP3942 contains current limit circuitry that protects the
device in the event of excessive output current and/or output
shorts to ground. Current is limited to 300 mA (typ) when the
output is shorted directly to ground. When the LP3942 is
current limiting, power dissipation in the device is likely to be
quite high. In this event, thermal cycling should be expected
(See Thermal Shutdown section).
Charge Pump Application
Information
OUTPUT VOLTAGE RIPPLE
The amount of voltage ripple on the output of the LP3942 is
highly dependent on the application conditions: output cur-
rent and the output capacitor, specifically. A simple approxi-
mation of output ripple is determined by calculating the
amount of voltage droop that occurs when the output of the
LP3942 is not being driven. This occurs during the charge
phase ( Φ1 ). During this time, the load is driven solely by the
charge on the output capacitor. The magnitude of the ripple
thus follows the basic discharge equation for a capacitor (I =
C x dV/dt), where discharge time is one-half the switching
period, or 0.5/FSW. Put simply,
A more thorough and accurate examination of factors that
affect ripple requires including effects of phase non-overlap
times and output capacitor equivalent series resistance
(ESR). In order for the LP3942 to operate properly, the two
phases of operation must never coincide. (If this were to
happen all switches would be closed simultaneously, short-
ing input, output, and ground). Thus, non-overlap time is built
into the clocks that control the phases. Since the output is
not being driven during the non-overlap time, this time
should be accounted for in calculating ripple. Actual output
capacitor discharge time is approximately 60% of a switch-
ing period, or 0.6/FSW.
The ESR of the output capacitor also contributes to the
output voltage ripple, as there is effectively an AC voltage
drop across the ESR due to current switching in and out of
the capacitor. The following equation is a more complete
calculation of output ripple than presented previously, taking
into account phase non-overlap time and capacitor ESR.
A low-ESR ceramic capacitor is recommended on the output
to keep output voltage ripple low. Placing multiple capacitors
in parallel can reduce ripple significantly, both by increasing
capacitance and reducing ESR. When capacitors are in
parallel, ESR is in parallel as well. The effective net ESR is
determined according to the properties of parallel resistance.
Two identical capacitors in parallel have twice the capaci-
tance and half the ESR as compared to a single capacitor of
the same make. On a similar note, if a large-value, high-ESR
capacitor (tantalum, for example) is to be used as the pri-
mary output capacitor, the net output ESR can be signifi-
cantly reduced by placing a low-ESR ceramic capacitor in
parallel with this primary output capacitor.
CAPACITORS
The LP3942 requires 4 external capacitors for proper opera-
tion. Surface-mount multi-layer ceramic capacitors are highly
recommended. These capacitors are small, inexpensive and
have very low equivalent series resistance ( ≤ 10 mΩ. typ.).
Tantalum capacitors, OS-CON capacitors, and aluminum
electrolytic capacitors generally are not recommended for
use with the LP3942 due to their high ESR, as compared to
ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LP3942. These capacitors have tight capacitance tolerance
(as good as ±10%), hold their value over temperature (X7R:
±15% over −55˚C to +125˚C; X5R: ±15% over −55˚C to
+85˚C), and typically have little voltage coefficient. Capaci-
tors with Y5V and/or Z5U temperature characteristic are
generally not recommended. These types of capacitors typi-
cally have wide capacitance tolerance (+80%, −20%), varies
significantly over temperature (Y5V: +22%, −82% over
−30˚C to +85˚C range; Z5U: +22%, −56% over +10˚C to
+85˚C range), and has poor voltage coefficients. Under
some conditions, a nominal 1 µF Y5V or Z5U capacitor could
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